1. Field of the Invention
This invention relates generally to controlling radio frequency (RF) transmission power and maximizing battery life in a portable communications device. More particularly, the invention relates to a low noise switching voltage regulator.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. While the different modulation and transmission schemes each have advantages and disadvantages, a common goal for all of these devices is maximizing the amount of time that a handset can operate on a single battery charge. This is referred to as maximizing the “talk time” of the handset.
Maximizing the talk time of a portable communication handset is typically the greatest challenge facing the designers of such devices. While many different approaches have been investigated, the single greatest improvement in talk time is generally achieved by reducing the power consumption of the RF PA. Typically, the RF PA consumes the greatest amount of power in a portable communication handset.
Many techniques have been implemented in an attempt to reduce the power consumption of the RF power amplifier. One such technique is the use of a switching voltage regulator to reduce the supply voltage supplied to an RF power amplifier. By reducing the voltage to the PA it can be operated closer to the saturation point, and hence the efficiency of the transmitter can be improved. There are two primary issues that should be resolved before this technique can be practically realized. The first issue has to do with the output noise or “ripple” of the switching supply that will be used to supply the RF PA. Any noise present on the supply of the RF PA will manifest as noise in the output RF spectrum of the RF PA. Most communications standards, such as the global system for mobile communications (GSM) have strict specification limits on the amount of noise the system is allowed to add to the transmitted frequency.
To comply with the power output spectrum standard for GSM, the power output of an RF power amplifier is tested against a specified power output specification, referred to as a power output spectral “mask.” The mask defines an RF spectrum within which the power output of the RF power amplifier must reside. Out-of-band emission, spurious emission and other factors are tightly controlled. If any portion of the output of the RF power amplifier violates the spectrum defined by the mask, the portable communication handset will fail the power spectral mask test and will not be permitted to operate in the communication system. Other communication standards, have similar power output mask specifications.
The second issue concerning RF PA saturation demands careful control of the RF PA supply signal such that the RF PA power saturation point is above the desired output power of the system. If these conditions aren't met, the power ramp-up and ramp-down required by GSM will allow RF power to migrate into adjacent channels and could cause the power amplifier to fail the switching transient specification such as those set forth by GSM standard 11.10.
Furthermore, in existing portable communication handsets that use a switching voltage regulator to supply voltage to an RF power amplifier, the efficiency of the power amplifier is limited due to lack of ability to reduce the ripple to an acceptable level.
One manner of reducing the ripple is to carefully select components in the switching voltage regulator to absorb and minimize the ripple. Unfortunately, this typically leads to unacceptably large and costly components.
Therefore, there is a need in the industry to further reduce power consumption and increase talk time in a portable communication handset.